1. Field of the Invention
The present invention relates to a zoom lens device for a camera, and more particularly to a zoom lens device capable of preventing the focal point from deviating under the influence of backlash of the zooming mechanism.
2. Related Background Art
Almost all recent compact cameras have a built-in zoom lens device. A two-group type optical system of the zoom lens is widely used in order to make the total length of the lens short and to make the camera compact. The two groups are a front lens group and a rear lens group. The front lens group has a variable power and focussing function, and the rear lens group has the function of compensating for deviation of the focal point upon moving the front lens group. During zooming, a movable lens barrel containing the front lens group and the rear lens group moves along the optical axis, and at the same time, the rear lens group moves within the movable lens barrel to change the distance between the front and rear lens groups. During focussing, only the front lens group moves within the movable lens barrel by an amount corresponding to the subject distance. Therefore, the position of the rear lens group is determined by the focal length, while the position of the front lens group is determined by the focal length and the subject distance.
As lens moving mechanisms, a cam mechanism and a helicoid mechanism are known. These mechanisms give rise to mechanical backlash, which shifts or causes to deviate the position of the movable lens barrel along the optical axis or slants it relative to the optical axis. Therefore, the subject distance to be focussed changes with the direction of movement, i.e. the zooming direction, of the movable lens barrel. In order to maintain the same conditions of the movable lens barrel irrespective of its zooming direction, the mechanical backlash should be kept as small as possible. However, this requires high precision machining operations, leading to high cost.
To reduce the influence of mechanical backlash of two members constituting the cam mechanism or the helicoid mechanism, the movable member is biased by a spring. When the movable member moves in the direction opposite to the spring force, a large force is necessary so that a motor of high power is required, resulting in high cost and an increase in battery consumption. A unidirectional approach method has been proposed (U.S. Pat. No. 4,936,664) in which without using a spring, the optical system is stopped at the end of movement in only one direction. More specifically, if the optical system moves in a first zooming direction (e.g., from telephoto to wide-angle), the optical system approaches directly and is stopped at a predetermined zoom position. However, if the optical system moves in the other zooming direction (from wide-angle to telephoto), the optical system slightly overruns a predetermined zoom position, and thereafter approaches and stops in this predetermined zoom position by movement in the first zooming direction. In the latter case, an operator senses a strange time delay of stopping of the lens system, after manipulation of a zooming operation lever. In addition, if the taking zoom lens and zoom finder are driven by a single motor, the finder magnification factor differs more or less from what the operator intended.